If it helps, I don't even think it was intentionally a 'let's make them lighter' thing, not even malice. I mean, you said yourself that Nito, Sheba, and Schera are popular because people find them attractive, and they find them attractive as they are in canon, with dark skin. So it'd be weird to intentionally draw them lighter skinned. It probably is just legitimately the artist not really knowing how to draw dark skinned people in bright lights.